Circuit for controlling a lighting unit having a periodic power supply with a thyristor

ABSTRACT

A circuit capable of receiving, in series with at least one light-emitting diode, a rectified A.C. voltage, comprising: a first gate turn-off thyristor connected to first and second terminals of the circuit; and a control circuit for turning off the first thyristor when the voltage between the first and second terminals exceeds a threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/797,924, filed Jun. 10, 2010, which claims the priority benefit ofFrench patent application number 09/53922, filed on Jun. 12, 2009, whichapplications are hereby incorporated by reference to the maximum extentallowable by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to lighting units with light-emittingdiodes intended to receive an A.C. supply voltage. It more specificallyrelates to circuits for powering such devices.

2. Discussion of the Related Art

For a long time, illumination devices have been formed based onincandescent light bulbs or on fluorescent tubes capable of receiving anA.C. supply voltage, for example, a 220-V mains voltage at 50 Hz. Morerecently, it has been desired to use light-emitting diodes. Such diodesespecially have a long lifetime and a high light output. They howeverrequire a power supply circuit capable of receiving the A.C. voltagefrom the mains.

Conventional circuits operate in linear mode, that is, they provide aD.C. voltage and a power adapted to the electrical characteristics ofthe diodes. The diodes are then maintained on for the entire duration ofeach halfwave of the mains voltage. This power supply mode has thedisadvantage of decreasing their lifetime. Further, linear power supplycircuits generally comprise high-voltage capacitors having thedisadvantage of being expensive and bulky.

SUMMARY OF THE INVENTION

An object of an embodiment of the present invention is to overcome allor part of the disadvantages of circuits for powering light-emittingdiodes.

An object of an embodiment of the present invention is to provide such acircuit improving the lifetime of the diodes.

An object of an embodiment of the present invention is to provide such acircuit which has low cost and is easy to form.

Thus, an embodiment of the present invention provides a circuit capableof receiving, in series with at least one light-emitting diode, arectified A.C. voltage, comprising: a first gate turn-off thyristorconnected to first and second terminals of the circuit; and a controlcircuit for turning off the first thyristor when the voltage between thefirst and second terminals exceeds a threshold.

According to an embodiment of the present invention, said circuitcapable of receiving, in series with at least one light-emitting diode,a rectified A.C. voltage, comprises a second thyristor connecting thegate of the first thyristor to said second terminal; and a firstresistive element connecting the gate of the first thyristor to saidfirst terminal or to a terminal of application of the rectified A.C.voltage.

According to an embodiment of the present invention, said circuitcapable of receiving, in series with at least on light-emitting diode, arectified A.C. voltage, comprises, in series with the first thyristor, avoltage dividing bridge for setting said threshold, the midpoint of thevoltage dividing bridge being connected to a gate of the secondthyristor.

According to an embodiment of the present invention, said circuitcapable of receiving, in series with at least one light-emitting diode,a rectified A.C. voltage, further comprises a circuit of temporary powerstorage between the midpoint of the voltage dividing bridge and saidgate of the second thyristor.

According to an embodiment of the present invention, said storagecircuit comprises: a second resistive element in series with acapacitive storage element, connecting said gate of the second thyristorto said second terminal; and a diode connecting the midpoint of thevoltage dividing bridge to said gate of the second thyristor.

According to an embodiment of the present invention, the resistivity ofsaid voltage dividing bridge is low as compared to the resistivity ofthe first resistive element.

According to an embodiment of the present invention, a capacitiveelectromagnetic disturbance attenuation element is connected betweensaid first and second terminals.

According to an embodiment of the present invention, the first thyristoris maintained on at the beginning of each halfwave of said rectifiedA.C. voltage, for a period ranging between 5% and 30% of the duration ofsaid halfwave.

An embodiment of the present invention further provides an illuminationdevice intended to receive an A.C. voltage comprising: a bridge forrectifying the A.C. voltage; at least one light-emitting diode; and acircuit capable of receiving, in series with at least one light-emittingdiode, a rectified A.C. voltage, according to any of the above-mentionedembodiments, series-connected with said at least one light-emittingdiode, between output terminals of said rectifying bridge.

According to an embodiment of the present invention, said circuitcapable of receiving, in series with at least one light-emitting diode,a rectified A.C. voltage, forms a dipole.

The foregoing objects, features, and advantages of the present inventionwill be discussed in detail in the following non-limiting description ofspecific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an illumination device with light-emittingdiodes;

FIG. 2 shows the simplified electric diagram of the illumination deviceof FIG. 1;

FIG. 3 shows the simplified electric diagram of the illumination deviceof FIG. 1; and

FIGS. 4A to 4G are simplified timing diagrams illustrating the operationof the illumination device of FIG. 3.

DETAILED DESCRIPTION

For clarity, the same elements have been designated with the samereference numerals in the different drawings. Further, the timingdiagrams of FIGS. 4A to 4G are not to scale.

FIG. 1 is a simplified view of an illumination device 1 withlight-emitting diodes 3.

FIG. 2 is a simplified electric diagram of device 1.

Device 1 comprises an assembly of light-emitting diodes 3 in series.Terminals A and C (FIG. 2) of diode assembly 3 are connected to a powersupply circuit 5 (POWER). In the shown example, terminals A and Crespectively correspond to the anode and cathode connection terminals ofthe assembly of diodes 3 in series.

Circuit 5 is capable of receiving an A.C. voltage V_(AC) (FIG. 2), forexample, the mains voltage, and of providing a power adapted to theelectrical characteristics of the assembly of diodes 3. Input terminalsof power supply circuit 5 are connected to terminals E and F of a base6. Base 6 may have any shape adapted to a connection on a socket, forexample, a screw thread. Other connections may be provided, for example,a direct wiring to a power supply connector. The entire device isassembled in a package 7 only leaving access to base 6 and diodes 3.Transparent glass, not shown, may protect diodes 3.

A switch 9 is generally provided, for example, between a terminal ofapplication of the phase of mains voltage V_(AC) and terminal E of base6, to control the powering-on of device 1. Switch 9 may correspond to awall switch that may be actuated by a user.

To improve the lifetime and the efficiency of the diodes, it is providedto maintain them on for a fraction only of each period of the mainsvoltage, which is sufficient, in relation with the eye's persistence ofvision, to guarantee a continuous illumination.

Thus, an aspect of an embodiment of the present invention is to providea power supply circuit capable of providing a pulse control signal, theelectric power received by the diodes depending on the duration of thepulse.

FIG. 3 is the electric diagram of an embodiment of the illuminationdevice of FIG. 2 showing power supply circuit 5 in more detailedfashion.

Terminals E and F of the base are connected to A.C. input terminals of afullwave rectifying bridge 20 capable of providing, between high and lowoutput terminals H and M, a rectified A.C. voltage V_(ACR). Terminal Mfor example corresponds to the reference voltage terminal of the circuitor ground. In the shown example, bridge 20 comprises four diodes 21, 23,25, and 27.

In this example, power supply circuit 5 further comprises a dipole orcontrol circuit 31 connected, in series with diode assembly 3, betweenoutput terminals H and M of rectifying bridge 20. Terminal A of diodeassembly 3 is connected to terminal H. Terminals I and J of dipole 31are respectively connected to terminal C of diode assembly 3 and toterminal M.

Circuit 31 comprises a gate turn-off thyristor 33 having its anodeconnected to terminal I. A voltage dividing bridge formed, for example,of two resistors 35 and 37 in series, is connected between the cathodeof thyristor 33 and terminal J. A resistor 39, of strong value withrespect to resistors 35 and 37, is connected between the anode and thegate of thyristor 33. Resistor 39 is used to turn on thyristor 33 at thebeginning of each halfwave of voltage V_(ACR). A cathode gate thyristor41 is forward connected between the gate of thyristor 33 and terminal Jfor controlling the turning off of thyristor 33. The voltage dividingbridge conditions the turning on of thyristor 41.

For the case where thyristor 41 would not be fast enough, a temporarypower storage circuit is preferably provided. This circuit, for example,comprises a resistor 43, series connected with a capacitor 45, betweenthe gate of thyristor 41 and terminal J, and a diode 47, forwardconnected between the midpoint of the voltage dividing bridge and thegate of thyristor 41.

Optionally, a capacitor 49 connects terminal I to terminal J, toattenuate electromagnetic disturbances linked to the switching ofthyristors 33 and 41.

Power supply circuit 5 may further comprise, between its input terminalsE and F, a varistor 29 of protection against possible overvoltages.

As a specific embodiment:

resistances 37 and 43 are on the order of a few tens of Ω, for example,on the order of 10 Ω;

resistance 35 is on the order of a few hundreds of Ω, for example, onthe order of 250 Ω;

resistance 39 is on the order of a few tens of kΩ, for example, on theorder of 75 kΩ;

capacitance 45 is on the order of a few hundreds of nF, for example, onthe order of 100 nF; and

capacitance 49 is on the order of a few tens of nF, for example, on theorder of 10 nF.

FIGS. 4A to 4G are simplified timing diagrams showing examples of thevariation of the voltages and currents at different points of theillumination device of FIG. 3. FIG. 4A shows the variation of rectifiedA.C. voltage V_(ACR). FIG. 4B shows the variation of current I₃₃ flowingthrough thyristor 33. FIG. 4C shows the variation of current I_(G41)flowing between the cathode gate and the cathode of thyristor 41. FIG.4D shows the variation of voltage V₄₁ across thyristor 41. FIG. 4E showsthe variation of current I₄₁ flowing through thyristor 41. FIG. 4F showsthe variation of current I_(LED) flowing through diode assembly 3. FIG.4G shows the variation of voltage V_(CM) between terminals C and M.

A steady state is assumed, that is, switch 9 is assumed to be on.

At a time t0 of beginning of a halfwave of voltage V_(ACR), thyristor 41is off. A current flows through diodes 3, resistor 39, the gate ofthyristor 33, and resistors 35 and 37. Thyristor 33 thus startsconducting. A conduction path is thus established between terminal A andthe ground, running through diode assembly 3, thyristor 33, andresistors 35 and 37 of low value of the voltage dividing bridge. Thediodes turn on. Current I_(LED) is then equal to current I₃₃.

From time t0, gate current I_(G41) of thyristor 41 increasesproportionally to current I₃₃, to within a factor especially dependingon the value of resistors 35 and 37 of the voltage dividing bridge. Acurrent for charging capacitor 45 further flows between the cathode ofdiode 47 and terminal M. Voltage V₄₁ across thyristor 41 is equal torectified voltage V_(ACR) minus the voltage drop caused by diodeassembly 3 and by resistor 39. Voltage V_(CM) is equal to rectifiedvoltage V_(ACR) minus the voltage drop of diode assembly 3.

At a time t1, current I_(G41) reaches a turn-on threshold I_(TH) ofthyristor 41. The turning on of thyristor 41 brings the gate ofthyristor 33 to ground (V₄₁=0 V), thus turning it off. Current I₃₃flowing through thyristor 33 thus becomes zero. There then is aconduction path between terminal A of diode assembly 3 and terminal M,running through diode assembly 3, resistor 39, and thyristor 41. At timet1, current I_(LED) becomes equal to current I₄₁. The value of resistor39 is selected to be sufficiently high for this current to be very low(it is shown as being zero in FIG. 4F). Thus, diodes 3 turn offsubstantially at time t1. The values of resistors 35 and 37 of thevoltage dividing bridge define the threshold of voltage V_(ACR) forwhich current I_(G41) reaches turn-on threshold I_(TH) of thyristor 41.The values of resistors 35 and 37 are for example selected so that thetime for which diodes 3 are on ranges between 5% and 30% of duration Tof a halfwave of rectified voltage V_(ACR). Further, at time t1, voltageV_(CM) is abruptly taken up to the value of voltage V_(ACR) (neglectingthe voltage drop in diodes 3 with respect to the voltage drop inresistor 39).

To ensure the priming of thyristor 41, capacitor 45 maintains a non-zerocurrent I_(G41) for some time after time t1.

Thyristor 41 remains conductive until the current that it conductscancels, that is, until end time t0+T of the halfwave. Thus, diodes 3are maintained off between times t1 and t0+T.

In a transient state of turning on of switch 9, this turning on mayoccur at any time of the halfwave. Thyristor 33 turns on at this timebut current I_(G41) immediately reaches turn-on threshold I_(TH) ofthyristor 41, thus turning off of thyristor 33 and turning off diodes 3until the beginning of the next halfwave. This enables the diodes to seeacross their terminals a voltage close to the maximum mains voltage fora very short time, thus avoiding their destruction. The diodes are thusprotected.

The use of a fullwave rectifying bridge provides a frequency of thediode control pulses equal to twice the frequency of the A.C. powersupply voltage. Such a frequency is sufficient to get rid of possibleflickering effects with a mains voltage of 50 Hz or 60 Hz.

An advantage of the provided circuit is that it has a low cost, a smallbulk, and is easy to form.

To form a power supply circuit capable of providing a pulse controlsignal, it could have been devised to use, instead of gate turn-offthyristor 33, a gate turn-on thyristor. This thyristor would then haveto be turned on at a time close to the end of each halfwave of therectified voltage, the diodes remaining substantially conductive untilthe end of each halfwave. However, spurious voltage peaks may appear, inparticular at the turning on of switch 9. Such peaks would be capable ofcausing the turning-on of the diode turn on thyristor. In this case, thediodes would remain on substantially until the end of the halfwave. Ifswitch 9 had been turned on, for example, at the beginning of ahalfwave, the diodes would receive a much greater power than that forwhich they have been provided, which would cause their destruction.

Specific embodiments of the present invention have been described.Various alterations and modifications will occur to those skilled in theart. In particular, the present invention applies whatever the availableA.C. supply voltage. Further, the number of light-emitting diodes andtheir connection may vary.

Further, the illumination device may comprise a dimming function if oneof the resistors of the voltage dividing bridge is replaced with avariable resistor.

Moreover, in the circuit described in relation with FIG. 3, terminals Aand C of the assembly of light-emitting diodes are respectivelyconnected to terminal H, corresponding to the high terminal of therectified supply voltage, and to anode terminal I of gate turn-offthyristor 33. According to an alternative embodiment, resistor 37 isconnected to terminal M, no longer directly but via diode assembly 3.According to another variation, resistor 39 is no longer connected toterminal C but to terminal H. Such variations may be combined.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

What is claimed is:
 1. An apparatus comprising: at least onelight-emitting diode; a first thyristor to regulate a flow of powerthrough the at least one light-emitting diode; and a control circuitadapted to produce a signal for input to a gate of the first thyristorto regulate conductivity of the first thyristor to make the firstthyristor conductive for a portion of a period of a power source byturning off the first thyristor when the control circuit detects an endof the portion of the period of the power source for which the firstthyristor is to be conductive; the control circuit comprising a voltagedivider having an intermediate node, and a second thyristor having aconductivity regulated based on a voltage at the intermediate node ofthe voltage divider.
 2. The apparatus of claim 1, wherein: an anode ofthe second thyristor is connected to the gate of the first thyristor;the signal that the control circuit is adapted to produce is a signalflowing through the second thyristor when the second thyristor isconductive.
 3. The apparatus of claim 1, further comprising: an inputterminal to receive an alternating current power signal from the powersource; and a rectifier to produce a rectified signal from thealternating current power signal, wherein: the control circuit regulatesthe conductivity of the first thyristor to make the first thyristorconductive for a portion of a half-wave of the rectified signal, thevoltage divider is arranged to receive the rectified signal, and thesecond thyristor is arranged to be conductive when a voltage at theintermediate node of the voltage divider exceeds a value.
 4. Theapparatus of claim 1, wherein the control circuit regulates theconductivity of the first thyristor to make the first thyristorconductive for a portion of a cycle of an alternating current powersignal.
 5. The apparatus of claim 4, further comprising: an inputterminal to receive the alternating current power signal from the powersource; and a rectifier to produce a rectified signal from thealternating current power signal, wherein the control circuit regulatesthe conductivity of the first thyristor to make the first thyristorconductive for a portion of a half-wave of the rectified signal.
 6. Theapparatus of claim 5, wherein the at least one light-emitting diode andthe control circuit are arranged to receive the rectified signal.
 7. Anapparatus comprising: at least one light-emitting diode; a firstthyristor to regulate a flow of power through the at least onelight-emitting diode; a voltage divider coupled to the first thyristorand having an intermediate node; and a second thyristor having aconductivity regulated based on a voltage at the intermediate node ofthe voltage divider.
 8. The apparatus of claim 7, wherein the first andsecond thyristors regulate light emission of the at least onelight-emitting diode such that the at least one light-emitting diodeemits light for a portion of a half-wave of a rectified alternatingcurrent signal.
 9. The apparatus of claim 7, wherein the first andsecond thyristors regulate a flow of power via the at least onelight-emitting diode such that power flows via the at least onelight-emitting diode for only the portion of the period of the powersource.